In the solar cell technology invented by the late Dr. Praveen Chaudhari, a method is disclosed (U.S. Pat. No. 9,054,249 B2) for making a tandem solar cell in which a “thin-silicon film can be used for heteroepitaxial deposition of other semiconductors, which might be more efficient converters of light to electricity.” The material “CZTS” or Copper Zinc Tin Sulfide is a quaternary semiconducting compound which has received increasing interest since the late 2000's for applications in solar cells. CZTS provides good optical properties and has a band-gap energy from approximately 1 to 1.5 eV depending on the degree of substitution of S with Se and a large absorption coefficient in the order of 104 cm-1. In August, 2012, IBM announced they had developed CZTS solar cell capable of converting 11.1% of solar energy to electricity. Recent material improvements for CZTS have increased efficiency to 12.0% in laboratory cells, but more work is needed for their commercialization. Properties such as carrier lifetime (and related diffusion length) are low (below 9 ns) for CZTS. This low carrier lifetime may be due to high density of active defects or recombination at grain boundaries.
If a tandem solar panel could reach 30 percent efficiency, the impact on the balance-of-system cost could be enormous: only two thirds of the number of panels would be needed to produce the same amount of power as panels that are 20 percent efficient, greatly reducing the amount of roof space or land, installation materials, labor and equipment. (Sivaram et al. “Outshining silicon . . . ”, Scientific American, 2015). The maximum efficiency for a two junction tandem under the AM1.5G spectrum and without concentration is 47%. At the peak efficiency the top cell has a bandgap of 1.63 eV and the bottom cell has a bandgap of 0.96 eV.
The theoretical limits to multi junction efficiencies for conversion with 1,2,3, and 36 bandgaps is 37, 50, 56 and 72% respectively. The improvement in efficiency from one to two bandgaps is considerable, but the returns diminish as more bandgaps are added. This is fortunate since the practicality of a device with five or more junctions is questionable. (Handbook of Photovoltaic Science and Engineering, Luque and Hegedus, p. 319).
The concept of the stacked solar cell was introduced to increase output voltage of a-Si:H solar cells. Only later it was recognized that stacked cells also offer a practical solution for improving the stabilized performance of a-Si:H based solar cells. Different terms such as tandem or dual junction or double junction solar cells are used in the literature to describe a cell in which two junctions are stacked on top of each other. A stack of three junctions is named a triple junction solar cell. The multi junction solar cell structure is far more complex than the single junction solar cell. For its successful operation there are two crucial requirements: (i) the current generated at the maximum power point has to be equal in each component cell (current matching) and (ii) an internal series connection between the component cells has to feature low electrical and optical losses. The internal series connection is accomplished at the p-n junction, where the recombination of oppositely charged carriers arriving from the adjacent component cells takes place. (M. Zeman “Advanced Amorphous Silicon Solar Cell Technologies”).
The requirement of current matching reflects the fact that component cells function as current sources which are connected in series. The component cell that generates the lowest current determines the net current flowing through the stacked two terminal cell. In order to avoid current losses, each component cell should generate the same current. The current generated by a component cell depends mainly on the absorption in the absorber layer of the cell, which is determined by the thickness of the absorber. Current is matched by adjusting the thickness of the absorber layer of each component cell. (M. Zeman “Advanced Amorphous Silicon Solar Cell Technologies”).
The tunnel recombination junction deals with the interface between the component cells. This interface is in fact a p-n diode. An ohmic contact between the component cells is required for proper operation of the stacked solar cell. The problem of obtaining the ohmic contact between the component cells can be resolved by fabricating a so-called tunnel recombination junction. This junction ensures that the electrons arriving at the n-type layer of the top cell and the holes arriving at the p-type layer of the bottom cell fully recombine at this junction. The recombination of the photogenerated carriers at this interface keeps the current flowing through the solar cell. A very high electric field in this reverse biased p-n junction facilitates tunneling of the carriers towards the defect states in the center of the junction. The effective recombination of the carriers takes place through these defective states. A tunnel recombination junction is usually realized by using microcrystalline silicon for at least one of the doped layers in order to obtain good ohmic contact. Another approach is to incorporate a thin oxide layer at the interface between the two component cells that serves as an efficient recombination layer. When the p-n junction functions as a good ohmic contact, the Voc of the stacked cell is the sum of the open circuit voltages of the component cells. (M. Zeman “Advanced Amorphous Silicon Solar Cell Technologies”).